Ink jet printing has become increasingly popular, particularly for so-called "desk-top publishing", because of its capability to produce small volumes of printed matter from digital input, at high throughput speeds. Recent equipment developments have led to the introduction of multi-color ink jet printers that integrate colored graphics and text. To some extent, however, the applications of ink jet printing have been limited due to the demanding requirements the inks must meet in order to provide high quality text and graphics.
Ideally, the ink used to print the text (generally black) will exhibit the sharp edge acuity needed to produce high print quality. One method of achieving sharp edge acuity is to select an ink that has relatively high surface energy; i.e., the ink will tend not to absorb into the printing media (generally paper), but leave the ink colorant (dye or pigment) on the media surface.
Another factor affecting ink selection is the tendency of inks to "bleed" when printed adjacent to one another. The problem of "bleeding" is particularly acute when the first ink has not yet dried when the next color is printed, which is the case in ink jet printing. One approach to minimizing bleed is to select inks having relatively low surface energy whereby the inks will rapidly absorb into the media, and thus be less affected by subsequent colors that are printed.
One approach to balancing the need for sharp edge acuity in the text, while minimizing bleed, has been to select a black ink exhibiting high surface energy for use with colored inks (typically cyan, magenta, and yellow) having low surface energies. Unfortunately, however, this approach is prone to produce a white or weakly colored region, referred to as "halo", at the interface of the black and colored inks. In some instances, one or more of the inks also may shrink back from their original printed areas. These tendencies of halo formation and shrinkage cause a loss of image optical density, and a loss of image quality, at the ink interface.
Thus, there is a need for improved inks that retain sharp edge acuity for printed text, have minimum bleed, and do not present the halo formation and shrinkage problems heretofore observed at the interface of black and colored inks.